Risperdal ConstaTM (risperidone) was the first long-acting depot formulation of an atypical antipsychotic belonging nowadays to standard treatments for schiz-ophrenic patients. The microsphere technology of this depot formulation re-sults in a complex pharmacokinetics with a delay (lag time) of relevant drug levels for 3 to 4 weeks, which therefore initially requires additional oral doses. The piecewise polynomial regression approach based on sign functions was used to define the complex concentration versus time curve after a single in-tramuscular injection. A 3-phase linear model with logarithmic transformation of concentration data was suited to fit the concentration time data and gave valuable data on both half-lives and phase borders (deflection points).